In the field of superconducting transmission lines, the transmission line cable and cable containment envelope or enclosure terminate at each end of the line so that the cryogenic cooling fluid for maintaining the line below its superconducting critical temperature can enter at one end of the cable enclosure and exit from the other end thereof. Referring to FIG. 1a of the above cited Forsyth et al. U.S. Pat. 3,902,000, which is incorporated by reference herein, 7.degree.K cooling fluid enters at one end, exits at the other end and returns to the refrigerator in a separate enclosure.